Coatings for motor vehicles, airplanes, rail-mounted vehicles and the like typically combine functions of decoration and protection. As such, the development of coatings having desired properties with respect to corrosion protection, scratch resistance, optics, coloring, chemical attack and resistance to a variety of other possible environmental effects is known. In addition, coatings can include multiple layers. For example, the “paint” on a motor vehicle can include a topcoat that has a base layer and a separately applied clearcoat on top of the base layer. The base layer is typically employed for color while the clearcoat provides protection of coating pigments in the base layer, scratch/mark resistance, gloss and/or depth of field.
The application of a coating having multiple separate layers can be costly and present technical challenges such as prolonged application time, increased equipment needs and the like. Therefore, a coating formulation that self-stratifies into, for example, a base layer and a top layer could streamline a coating process, reduce manufacturing costs, etc., and thus would be desirable.
Regarding a base layer, organic/inorganic hybrid materials have received much attention for more than two decades [1, 2], since the hybrids synergistically combine the advantageous properties of both materials. The hybrid materials provide unique properties such as improved physical, mechanical, thermal, gas barrier, and photonic properties [3-7]. A variety of elastomers, thermoplastics, and crosslinked systems have been modified in situ with inorganic materials [8-10]. The hybrid materials have combined the properties of the inorganic materials, i.e. hardness, durability, and thermal stability, and organic polymers, i.e. flexibility and toughness. As a consequence, such hybrids are promising materials for various applications, such as solid state lasers, replacements for silicon dioxide as insulating materials in the microelectronic industry, contact lenses or host materials for chemical sensors [11-14].
Coatings science has also made improvements in corrosion protection, impact, chemical, tamper resistance, antifouling, appearance, flexibility, and impermeability by the application of inorganic/organic hybrid coating systems [15-18]. In the hybrids, the sol-gel technique of alkoxysilanes is one of the useful methods to prepare organic/inorganic hybrid materials, since the reaction can proceed in liquid solution at ambient temperature. The general sol-gel reaction scheme is based on the hydrolysis of various alkoxides to form respective silanols [19]. This is followed by a condensation reaction occurring between silanols or silanols and alkoxides. The organic components of the inorganic-organic hybrids can be, in general, generated either by simultaneous synthesis of two independent (not covalently-bound) polymer networks (organic and inorganic), or by creation of matrices with covalent bonds connecting the organic and inorganic components [20, 21]. Organic monomers or polymers modified with alkoxysilane groups are used as coupling agents to provide bonding to the in situ formed inorganic structure. Strong interaction between organic and inorganic phases has been found to improve the mechanical properties of the hybrid [20, 22, 23].
Silicon sol-gel techniques have been widely used to prevent the corrosion of metals and to improve the coatings adhesion [24-27]. Holmes-Farley and Yanyo [28] used tetraethoxysilane (TEOS) in conjunction with an aminosilane adhesion promoter to prevent corrosion on aluminum substrate. Soucek et al. [29, 30] studied polyurea and polyurethane organic/inorganic films using different sol-gel precursors such as organofunctional alkoxysilanes. The polyurethane/polysiloxane was developed to be a “Unicoat” system [31-33]. In this system, polyurethane provides the general mechanical properties as both the primer and topcoat, and polysiloxane functions as an adhesion promoter and corrosion inhibitor. The ceramer films exhibited enhanced adhesion and corrosion resistance properties via a self-assembly phase separation mechanism. The corrosion resistance was comparable to chromate pretreated systems, and thus part of the body of research devoted to chromate replacement. Organic/inorganic hybrid coatings were also reported mixing drying oils with sol-gel precursors, using an approach developed by Soucek and coworkers [34, 35]. The resulting hybrid coatings showed improved hardness and adhesion with increasing sol-gel precursor content.
There have been few reports to date on the preparation of epoxide resin/silica hybrids. Several researchers [36-38] investigated epoxide resin-montmorillonite hybrids, using the intercalation process and the well-defined dimensions of the clay layers. Landry et al. [39] prepared a hybrid material from a very high molecular weight epoxide, functionalized with γ-aminopropyltriethoxysilane, and silica. Hussain et al. [40] reported the preparation of a hybrid material based on an epoxy resin/silica system, using tetraglycidyl-meta-xylene-diamine as the resin. In their study, the hybrid was prepared via producing silica filler, using sol-gel method, which was subsequently incorporated into the epoxy resin mixture. The epoxide-silica interpenetrating networks (IPNs) were also investigated by Bauer et al. [41] and modeled by Matejka et al. [42, 43]. The hybrid systems composed of organic rubbery network and inorganic silica structure formed by the sol-gel process from tetraethoxysilane.
Epoxides, in particular bisphenol-A type (BPA) epoxides, have been the primer of choice for metal since its introduction into the commercial marketplace. Epoxide primers have excellent adhesion to metal due to the secondary hydroxyl group in the repeat unit [44]. Epoxides are also noted for hardness, hydrophobicity, and chemical resistance due to the BPA group. The systematic characterization, evaluation and comparison of the corrosion performance and adhesion for low molecular weight epoxide derivatives/tetraethoxysilane oligomer hybrid systems have not yet been reported.
Regarding a top layer, fluoropolymers are considered an ideal solution for coatings applications requiring chemical resistance (to acids, bases, solvents, and hydrocarbons), high thermal stability [45], low friction [46], and excellent weatherability. The unique combination of optical and electrical characteristics, low dielectric constant, low dissipation factor [47], and low surface energy [48, 49] has also led to growing interest in fluorine chemistry for a wide range of applications. In addition to the fluorinated olefin-based polymers, step growth fluoropolymers have been developed to obtain similar performance characteristics as well as to expand the potential scope of coatings applications. Even low fluorine content results in substantial beneficial properties [50]. Acrylics are non-yellowing and resist chemicals, i.e. gasoline, salt, oil, anti-freeze. Thus, in commercial coatings, fluorinated acrylics are used in the automotive industry, especially for automotive clearcoat formulations.
Fluoroacrylic copolymers have been extensively researched to discover applications in optics [51, 52], electronics [53], and construction (protective [54-56] and high performance coatings [57]). There have been various reports of fluoroacrylates prepared by emulsion polymerization [58-62], atom transfer radical polymerization [63-65], and high radiation polymerization [66, 67]. Furthermore, fluorinated methacrylates have been investigated for the synthesis of fluoropolymers with reduced polymerization shrinkage and improved strength [68]. The effect of fluorinated monomers on reduction of surface energy and surface wettability has also been published elsewhere [69, 70].
A number of fluorinated coatings have also been recently reported. Wynne et al. [71] focused on the surface modification of polyurethanes with short fluorinated side chains. The fluorinated groups improved the hydrophobicity, while retaining the bulk properties of a conventional polyurethane. They also demonstrated that the effectiveness of antimicrobial coatings was dependent on the nature of both fluorinated side chains and quaternary alkylammonium side chains [72]. Ober et al. [73] reported the antifouling coatings based on both hydrophobic (fluorinated) and hydrophilic functionalities as surface domains. They explored the marine organisms' settlement behavior on surface domains with distinct wettability. Delucchi et al. [74] studied the fluoropolyether coatings based on perfluoroether oligomeric diols cured with isocyanates. They concluded that fluorine content is not always the dominating parameter since other physical factors, such as the crosslink density, phase separation, and glass transition temperature, can play major roles on coating performance.
Solventborne high-solids acrylic technology is still widely used in the coatings of automotive and general industrial plastics. The primary advantages of solventborne acrylic technology are adhesion, quick drying, and durability [75]. On the other hand, the requirements of several government regulations have resulted in the product development to improve the environment, which is one of the main drivers in the coatings industry. Therefore, high-solids acrylics have been the subject of continuing research [76-80]. However, a very comprehensive study on synthesis and characterization of fluorinated acrylic copolymers for high-solids coatings has not yet been reported. Since the coating industry still relies on conventional free radical-initiated polymerization for the production of acrylics, it is important to obtain high-solids (60 wt. %) surface active acrylics with moderate polydispersity by a technique capable of economically producing functional acrylic copolymers.